[clang] [Clang] Fix constexpr inc/dec operators for OverflowBehaviorTypes (PR #183826)

[Justin Stitt via cfe-commits]

JustinStitt wrote:

@tbaederr 
> The new diagnostic seems like a step back. And there are no new tests.

Does the wording in [`586fd4a`](https://github.com/llvm/llvm-project/pull/183826/commits/586fd4a63c790b8b7c3bacb61e2815b3b54a1766) sound better?

I added more constexpr tests with [`ed76cf5`](https://github.com/llvm/llvm-project/pull/183826/commits/ed76cf5e3096ada058cf9b9248b73429a7168d27)


https://github.com/llvm/llvm-project/pull/183826